1. Field of the Invention
This invention relates generally to a method and apparatus for producing images of the interior of a body, such as a human body, using data derived from NMR measurement techniques, and more particularly to a method and apparatus for synthesizing computed images based on a minimal number of such measurements.
2. Description of the Prior Art
Nuclear magnetic resonance (NMR) is a physical phenomenon which has been used in recent years to produce images of the interior portions of bodies, particularly human bodies, for diagnostic and other purposes. The images produced using NMR techniques are particularly useful when the interior of the body or body portion contains a variety of relatively soft tissues but can be employed under a rather wide variety of circumstances. Such images have been compared with those obtained by radiographic techniques, but NMR images portray different characteristics and, particularly, some characteristics which are not easily portrayable by radiographic or other imaging methods, if at all.
An excellent fundamental description of basic NMR physics and imaging techniques is found in Kaufmann et al, Nuclear Magnetic Resonance Imaging in Medicine, Igaku-Shoin Medical Publishers, Inc., New York and Tokyo (1981). There are some features of NMR and of common techniques for producing NMR images which are particularly important in the present context and will be reviewed.
In NMR examinations, the body being examined is subjected to one magnetic field which is usually constant in magnitude and another field which lies along at least one different vector from the first and is usually time-varying, the exact characteristics of these fields being a function of choice between any one of several available imaging techniques. In any such technique, there are characteristics of the field energy which must be selected in advance. In spin echo imaging, for example, one selects the repetition time T.sub.R which is defined as the time between successive alternations in a transverse field, and the sampling delay time .tau. (Tau). The signals resulting from the impositions of the fields in accordance with the selected times are detected and stored or recorded. The levels of these signals, correlated with their physical positions, can be represented by a matrix of numbers and the numbers can, in turn, be displayed as a matrix of points or pixels having different light or dark levels, the composite of these pixels being usable to form an image wherein the differences appear as differences in contrast.
The contrast in an image is extremely important because it is that contrast which permits the examiner, such as a physician, to observe and analyze the "slice" of the body of which the image was made and, in a medical context, to thereby form a diagnosis. The degree of contrast is a function of the T.sub.R and .tau. values which are selected before the measurements are made and also of the intrinsic properties of the materials including net magnetization M.sub.o (which is proportional to proton density P), and the relaxation times T.sub.1 and T.sub.2. However, a set of T.sub.R and .tau. values will, in most cases, produce an image with excellent contrast between certain sets of materials but insufficient contrast between other materials. It is therefore necessary to make numerous sets of measurements with various and T.sub.R and .tau. values and to observe the images resulting from those measurements in order to be able to adequately examine the various tissues involved.
As will be recognized, this is a time-consuming process in which the patient is often subjected to discomfort and, in addition, is repeatedly subjected to the effects of a rather strong, unidirectional magnetic field. For certain measurements, body movement must be prevented during each measurement, the duration of which may last in the order or seconds or minutes, depending upon the pulse sequence technique being used and the materials under observation. These conditions establish rather severe limitations on the length of time which can be taken for such measurements, and therefore, the number of measurements which can be made. Furthermore, it is not always possible for the examining physician to recognize in advance or at the time of the measurements which materials are most significant in the images.
Additional background information and disclosures of devices and techniques in the field to which this invention relates can be found in the following articles and U.S. patents.
Wehrli, F. R., J. R. MacFall, and G. H. Glover. The dependence of nuclear magnetic resonance (NMR) image contrast on intrinsic and operator-selectable parameters. Presented at the meeting of the SPIE, Medicine XII, Volume 419, April, 1983.
Young, I. R. et al. Contrast in NMR Imaging. Presented at the Society of Magnetic Resonance in Medicine, August, 1983.
Ortendahl D., et al. Calculated NMR images. Presented at the Society of Magnetic Resonance in Medicine, August, 1983.
Davis, P. L., et al. Optimal spin echo images for liver lesions by retrospective calculations. Presented at the Society of Magnetic Resonance in Medicine, August, 1983.
______________________________________ U.S. Pat. No. Inventor ______________________________________ 3,789,832 Damadian 4,045,723 Ernst 4,284,948 Young 4,292,977 Krause et al 4,297,637 Crooks et al 4,307,343 Likes 4,318,043 Crooks et al 4,354,499 Damadian 4,355,282 Young et al 4,390,840 Ganssen et al ______________________________________